1. Field of the Invention
This invention relates to a video camera with an automatic focus adjusting mechanism, which effects automatic focus adjustment of an image focusing optical system for focusing an image of an object on an image pick-up plane by emitting measuring light from a light emitting section and measuring the distance of the object from a lens by detecting the measuring light reflected by the object in a light receiving section, and more particularly to a portable video camera with a handgrip for carrying it.
2. Description of the Prior Art
There have heretofore been video cameras with an automatic focus adjusting mechanism for automatically adjusting the focus of an image pick-up lens by measuring the distance between the video camera body and an object to be picked up. The automatic focus adjusting mechanism drives an image pick-up lens focusing mechanism according to distance information obtained in a distance measuring section which is usually based on a light objection system using principles of the triangular surveying or a two-fold image coincidence system.
With the distance measuring section using triangular surveying principles, it is necessary to provide a long base line for triangular surveying in order to obtain sufficient precision for measuring distances. In addition, to avoid problems with parallax, the optical axis of the distance measuring system and the optical axis of the image pick-up system must coincide with each other.
Portable video cameras include those of the popular handheld type and shouldering type, which comprises a small size and light weight video camera body with a handgrip attached thereto for easy operation.
FIG. 1 is a perspective view of a well-known video camera of the shouldering type with an automatic focus adjusting mechanism.
The prior art video camera shown in FIG. 1 comprises a camera body 1, which is secured to a shouldering frame 2. The user may take pictures using this camera in a stable posture by shouldering a shouldering section 3 of the frame 2 and gripping a handgrip 4 projecting from the front of the frame 2.
The camera body 1 has a cylindrical lens holder 7 projecting from the front and supporting an image pick-up lens 6. The cylindrical lens holder 7 also supports a focus adjusting ring 8, which permits focus adjustment by the operation of an automatic focus adjusting mechanism not shown, which is accommodated in a housing 9 provided under the lens holder 7, or by manual operation. The housing 9 has two distance measurement windows or apertures 11 and 12 formed on the front. As shown in FIG. 2, light from an object, which enters the windows 11,12 respectively, is focused on a charge coupled device CCD line sensor 18 through reflecting mirrors 13 and 14, image focusing lenses 15 and 16 and a reflecting prism 17. The light from an object which enters through the window 11 is focused on and forms a reference image focused on the CCD line sensor 18 at a predetermined position thereof, while the light from the object which enters through the other window 12 forms a movable image on the CCD line sensor 18, which movable image is movable along the CCD line sensor 18 according to the distance of the object. The reference and movable images focused on the CCD line sensor 18 are used to calculate the distance of the object in order to control a focusing motor for automatic focus adjustment.
In the usual video camera with an automatic focus adjusting mechanism, erroneous operation thereof is liable to result in cases where the object is beyond the distance range (i.e., image angle range) measurable by the automatic focus adjusting mechanism or where a low contrast object is to be picked-up or where background and foreground scenes coexist. Due to these automatic focus problems, it is possible to adjust the focus manually as well. In the prior art video camera of the structure shown in FIG. 1, the housing 9 of the automatic focus adjusting mechanism provided under the lens holder 7 of the image pick-up lens 6 obstructs the manual operation of the focus adjusting ring 8, so that the operability is very inferior. Besides, there occurs parallax problems since the optical axes of the distance measuring system and image pick-up system do not coincide. Furthermore, because the prior art camera does not provide a long reference line for triangular light measurement, the precision of distance measurement is low.